Wastewater lift Stations or Pump Stations are used to “lift” liquid uphill—against gravity. They are an essential component of a wastewater collection system which takes wastewater from residents and businesses in a community and pumps it to a sewage treatment plant.
Managing a network of wastewater lift stations that pumps to the treatment plant is challenging. Accidental discharge of sewerage is a serious environmental hazard. Pumps can block, level sensing devices can fail, pipes can crack, lightning or power surges can damage control equipment, and power to control pumps can be interrupted. Discharge of gases is also problematic. Not only are the discharged gases pollutants, but some are quite odoriferous. Thus, nearby residents and businesses may encounter extremely unpleasant odors originating from a nearby lift station. Foul odors escaping the wet well or force main air space has been a continual problem over the years, particularly in residential neighborhoods.
A force main is a pressurized main pipe that can carry water, sewage, and other materials to and from wastewater lift stations. Many force mains have a high point equipped with a relief valve to allow gas trapped between upstream and downstream liquid to vent to the atmosphere.
Odors are inevitable. Wastewater undergoing storage, transportation, and treatment will produce odors. The production of odors comes as a result of wastewater composition and depends on the environment and the time that the particular environmental conditions have existed. During some conditions, odors will escape through vents.
Gaseous emissions may include hydrogen sulfide (H2S), ammonia (NH3), carbon dioxide (CO2), and methane (CH4). Some other gases resulting from biological activity are nitrogen (N2), oxygen (O2), and hydrogen (H2). Most common sewer gases are odorless except for hydrogen sulfide, which has the strong odor of rotten eggs, and ammonia, which has the odor of concentrated urine. The principal odor-causing gas in a wastewater lift station or force main is hydrogen sulfide.
Many methods have been used over the years to control or mask the foul odors including: 1) feed of chemicals such as hydrogen peroxide into the liquid; 2) aeration of the liquid; 3) chemical scrubbers; 4) biological scrubbers; 5) oxidation in the wet well or force main air space; and 6) activated carbon absorption. Chemicals can be extremely expensive. Several of the methods are not energy efficient or cost effective. Maintenance can also be expensive and cause excessive downtime of equipment. A cost effective odor reduction system that does not require replenishment of chemicals or a substantial capital investment is needed.
Further complicating matters, the volume and rate of odoriferous gases emitted can vary considerably based upon various factors, including temperature, dissolved oxygen, pH and well conditions such as volume. Hydrogen sulfide production increases at higher wastewater temperatures, and hydrogen sulfide released from wastewater will combine with water vapor to form sulfuric acid, which attacks sewers, manholes, wet well or force mains, and other facilities. The presence of certain types of gases in sewers depends on oxygen availability. When little oxygen is present, biological degradation is through anaerobic processes and the predominant gases released will be hydrogen sulfide and methane. Another condition that promotes the release of hydrogen sulfide gas is wastewater pH. Sulfide exists in wastewater in the ionic form above pH 7.5. Below pH 7.5, sulfide leaves the wastewater in the gaseous form. Yet another condition is the volume of wastewater in a well and vapor pressure. When the wastewater level falls due to pumps pumping the liquid from the wet well or force main, foul odor gases are not discharged and fresh air is sucked into the wet well or force main. However, when wastewater level rises due to increased wastewater production or precipitation, odoriferous gases are expelled. Thus, an effective odor reducing system must adapt to such changing conditions.
The system described in U.S. Pat. No. 8,337,759, while effective for reducing malodorous emissions, requires expensive equipment and frequent maintenance, and consumes considerable energy to power the oxygen concentrator and ozone generator.
Some other systems use pumps or fans to move odoriferous gases from the wet well or force main ullage through a deodorizing system. These systems operate even when pressure in the ullage of the wet well or force main does not exceed ambient pressure. Such systems expend considerable energy treating gases that would not otherwise escape the wet well or force main. They also expose expensive equipment with moving parts to corrosive and fouling gases and vapors.
Some other systems introduce oxidizers into the wet well or force main. Equipment (e.g., pumps), hardware and seals may undergo corrosion at an accelerated rate in the presence of the strong oxidizers.
Some other systems pass the odoriferous exhaust gases through chemical reactants (e.g., activated carbon). While effective for reducing odors, such reactants require periodic replacement. The replacement material and services can be costly.
What is needed is a system that allows odoriferous gases to reside in a wet well or force main until the pressure in the wet well or force main forces the gases out. The system should avoid locating any pumps or fans in the exhaust gas flow stream. The system should also allow the wet well or force main to draw in ambient air when the pressure in the wet well decreases appreciably below ambient pressure.
The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.